[PATCH] init/main: Clear boot task idle flag

[Liam R. Howlett]

Initial booting is setting the task flag to idle (PF_IDLE) by the call
path sched_init() -> init_idle().  Having the task idle and calling
call_rcu() in kernel/rcu/tiny.c means that TIF_NEED_RESCHED will be
set.  Subsequent calls to any cond_resched() will enable IRQs,
potentially earlier than the IRQ setup has completed.  Recent changes
have caused just this scenario and IRQs have been enabled early.

This causes a warning later in start_kernel() as interrupts are enabled
before they are fully set up.

Fix this issue by clearing the PF_IDLE flag on return from sched_init()
and restore the flag in rest_init().  Although the boot task was marked
as idle since (at least) d80e4fda576d, I am not sure that it is wrong to
do so.  The forced context-switch on idle task was introduced in the
tiny_rcu update, so I'm going to claim this fixes 5f6130fa52ee.

Link: https://lore.kernel.org/linux-mm/87v8cv22jh.fsf@mail.lhotse/
Link: https://lore.kernel.org/linux-mm/CAMuHMdWpvpWoDa=Ox-do92czYRvkok6_x6pYUH+ZouMcJbXy+Q@xxxxxxxxxxxxxx/
Fixes: 5f6130fa52ee ("tiny_rcu: Directly force QS when call_rcu_[bh|sched]() on idle_task")
Cc: stable@xxxxxxxxxxxxxxx
Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
Cc: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
Cc: Andreas Schwab <schwab@xxxxxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Peng Zhang <zhangpeng.00@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: "Mike Rapoport (IBM)" <rppt@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
---
 init/main.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/init/main.c b/init/main.c
index ad920fac325c..f74772acf612 100644
--- a/init/main.c
+++ b/init/main.c
@@ -696,7 +696,7 @@ noinline void __ref __noreturn rest_init(void)
 	 */
 	rcu_read_lock();
 	tsk = find_task_by_pid_ns(pid, &init_pid_ns);
-	tsk->flags |= PF_NO_SETAFFINITY;
+	tsk->flags |= PF_NO_SETAFFINITY | PF_IDLE;
 	set_cpus_allowed_ptr(tsk, cpumask_of(smp_processor_id()));
 	rcu_read_unlock();
 
@@ -938,6 +938,8 @@ void start_kernel(void)
 	 * time - but meanwhile we still have a functioning scheduler.
 	 */
 	sched_init();
+	/* Avoid early context switch, rest_init() restores PF_IDLE */
+	current->flags &= ~PF_IDLE;
 
 	if (WARN(!irqs_disabled(),
 		 "Interrupts were enabled *very* early, fixing it\n"))
-- 
2.39.2